Production of heat-treated sheets



United States Patent 3,196,053 PRQDUQTIQN SF HEAT-TREATED SHEETS John M.Hodge, Pleasant Hiils, Pm, assignor to United States Steel Corporation,a corporation of New .lersey N0 Drawing. Filed Aug. 13, B62, Ser. No.210,230 3 tCiairns. (Cl. res-r24 This application is acontinuation-in-part of my application Serial No. 847,445, filed October20, 1959, now abandoned, for Production of Heat-Treated Sheets.

This invention relates to the production of heat-treated sheets, andmore particularly, to the production of lowalloy heat-treated sheetscharacterized by a degree of flatness comparable to that obtained innonheat-treated steels.

The superior properties in respect to strength, ductility and toughnessof steels with microstructures consisting predominantly of temperedmartensite or lower bsinite is generally recognized and well known tothose skilled in the art. The usual practice for heat-treating steels insheet form to such a microst ucture involves heating the sheet to anaustenitizing temperature, cooling it to a temperature below that atwhich transformation to martensite is complete at a cooling rate fastenough to essentially prevent transformation during cooling attemperatures above those at which transformation to martensite or lowerbainite occurs, and tempering to the desired strength level. This heattreatment can be done either continuously with the sheet steel in stripform, or sheets of the desired final size can be heat treated singly.The continuous method is, however, preferred for quantity production.Althou h this. method of heat treatment is entirely satisfactory as amethod of producing sheets, i.e. .125" maximum gauge with the desiredmicrostructure and mechanical properties, the drastic water quenchrequired to insure the low-temperature transformation in steels ofrelatively low-alloy content and hardenability leads to a seriousdistortion in the heat-treated product, both from temperature gradientsduring cooling, and from stresses set up when transformation occurs atdifferent times alon with these temperature gradients. Since theheat-treated sheet is characterized by a relatively high yield strength,i.e. in excess of 75,000 psi, the removal of these distortions, and theproduction of a flat final sheet product by conventional flatteningmethods such as roller straightening, stretch straightening, or pressflattening is very ditficult and time-consuming and requires heavy,high-capacity equipment. These conventional methods, furthermore, allnecessitate the occurrence of localized plastic flow in order to producea fiat product, and this iresults in the development of residualstresses in the product which may be harmful. lthough these stresses maybe relieved by a thermal stress-relieving heat treatment, this involvesan additional operation and additional expense. The distortion occurringin heat-treatment can be minimized by various methods of controlling thecooling so that the resultant temperature gradients are minimized, but,in addition to the difficulties involved in establishing coolingpractices which will insure a suitably fiat product, particularly in acontinuous operation, these methods involve a relatively slow coolingrate from the austenitizing temperature and, therefore, require the useof steels of correspondingly higher hardenability, alloy content, andcost to insure complete transformation to martensite or lower bainiteafter cooling.

It is accordingly an object of this invention to provide a method ofproducing flat heahtreated sheets with microstructures of temperedmartensite or lower baiuite in an efiicient and economical manner.

It is a further object of this invention to provide fiat heat-treatedsteel sheets with microstructures of tempered martensite or lowerbainite and of relatively low-alloy content which require drasticquenching to produce the desired minimum 75,000 p.s.i. yield strength.

The method of this invention utilizes strip or sheet material, i.e.,.250" maximum thickness that has been heat-treated by drasticallyquenching from above its A temperature, such as by water quenching at arate in excess of F. per second. Such heat-treated sheet material,preferably in coil form to take full advantage of quantity production,is tempered in accordance with usual practice. According to theteachings of this invention, the heat-treated sheet material is thencold rolled sufficiently to remove all distortions resulting fromrolling, so that the .rip is returned to a substantially flat condition,and, at the same time, the surface smoothness and the thicknessuniformity is improved. The cold-rolled material is then tempered at atemperature higher than its recrystallization temperature but below thatof the original heat treatment. This recrystallizing or re-temperingtreatment restores the original heat-treated mechanical properties; and,since no further distortion occurs in the final tempering or recrystallizing treatment, a flat product having the desired strength andductility results.

Th-equenched and tempered low-alloy steels with tempered martensitic orlower bainitic microstructures which are applicable to the method ofthis invention are characterized by requiring drastic quenching, i.e. ata rate in excess of 100 F. per second from above their A temperatures toachieve a microstructure consisting essentially of martensite or lowerbainite. These low-alloy steels in the quenched and tempered conditionwill have yield strengths in excess of 75,000 psi. and relatively lowrate of work hardening. Because of their low workhardening rate they canWithstand extensive cold reduction by rolling without strain hardeningto the extent that a serious embrittlement would occur. This permitscold rolling reductions of 10 to 50% which are required to flatten thedrastically distorted sheets. Thus the quality of withstanding suchdrastic cold reductions permits removing the severe distortionsresulting from the drastic quenching required to achieve the desiredmicrostructure of tempered martensite or lower bainite in the economicallow-alloy steel.

The yield strengths of the quenched and tempered steels applicable tothe method of this invention are much higher and the rate of workhardening is much lower than those of the steels which are cold rolledin conventional production practices. These latter steels usually havemicrostructures of ferrite and pearlite, and are characterized by muchhigher work-hardening rates than those of the steels with temperedmartensitic or lower bainitic microstructures. Thus in the conventionalusage of cold rolling for the production of sheet products, the primaryobjective is to strengthen the material through work hardening, and thematerial before cold rolling i necessarily of a relatively low yieldstrength. In the method of this invention, on the other hand, theprimary objective of cold rolling is to obtain a flat product, and thelow rate of work hardening makes it possible to do this at therelatively high yield-strength levels of 75,000 to 150,000 p.s.i.,characteristic of these quenched and tempered steels.

The use of heavy cold rolling for flattening which is an essentialfeature of this invention offer the following advantages over prior artmethods:

1) Very severe distortion can be removed, and therefore, a drasticquench can be used in the heat treatment prior to flattening. Thispermits the use of economical low-alloy low-hardenabil'ity steels.

(2) Due to the heavy cold rolling reductions, the entire cross-sectionflows plastically in the rolling process so that residual stresses arenot created by localized strains such as may occur in prior art methodsof flattenmg.

(3) The cold rolling results in an improvement in surface quality anduniformity of thickness in the final product.

(4) The final step in the method of this invention, that of reheatingthe cold-rolled fiat product to a temperature above itsrecrystallization temperature but lower than the tempering temperatureused in the original heat treatment, results in a recrystallization ofthe cold-worked ferrite and a restoration of the original heat-treatedproperties.

The method is applicable to steels that (1) can be heat treated to amicrostrncture consisting substantially of tempered martensite or lowerbainite; (2) have a sulficiently low-alloy content that they requiredrastic quenching to produce a microstructure consisting essentially ofmartensite or lower bainite and (3) has a recrystallization temperatureafter cold rolling which is lower than the tempering temperature used inthe original heat treatment to achieve the desired strength level.Carbon content of the steel is limited by practical considerations to amaximum of about 0.25 to 0.30%, since higher carbon contents will causethe steel to harden to such an extent that the power requirements forcold reducing become uneconomical. A minimum of .10% carbon is requiredto achieve the desired strength. Thus, steels containing about 0.15 to0.25% carbon are preferred.

While numerous steel compositions will provide the desired combinationof properties, a preferred combination is that disclosed in UnitedStates Patent No.

Percent Carbon to .20 Manganese .60 to 1.0 Silicon .15 to .35 Nickel .70to 1.0 Chromium .40 to .80 Molybdenum .40 to .60 Vanadium .03 to .10Copper .0 to .50 Boron .002 to .006

With the balance iron and residual amounts of other elements.

In practicing the teachings of my invention, a heat of steel of thefollowing composition was made:

Percent Carbon .14

Manganese .86 Silicon .28

Nickel .82

Chromium .5 3 Molybdenum .5 1 Vanadium .05 Copper .3 1 Boron .003

' The steel of this composition was hot rolled to a coil of strip 49inches wide with a thickness of .10" and transformed to a martensiticstructure by heating it to about 1810 F. to austenitize it in a catenaryfurnace through which it traveled at about 10 feet per minute and thenwater quenching it. This resulted in considerable distortion in thestrip. After quenching, the strip was recoiled and tempered by holdingit at a temperature of about 1085 F. for about 4 hours in a boxannealing furnace.

The mechanical properties of the quenched and tempered strip were asfollows:

cases The strip was then flattened by cold rolling it to reduce thethickness to .05", i.e. about 50% reduction and again tempered in a boxannealing furnace by holding for 4 hours at a temperature between 1065to 1085 F. This temperature is above the recrystallizing temperature ofthe cold-rolled strip and slightly below the prior annealingtemperature. At this stage, the strip was quite fiat except for somecoil set and had the following mechanical properties:

Table II Yield Tensile Elongation Strength, Strength, in 2", 2% Otlset,p.s.i. Percent p.s.i.

Longitudinal 139, 700 145, 500 11. 0 Transverse 151, 600 153, 9. 0

From this it is seen that the material after flattening andrecrysta-llizing had substantially the same mechanical properties as theheat-treated material. To remove coil set, it was cold rolled to producea reduction of about .004" (i.e., just sufficient to remove coil set),out into sheets and tempered at a temperature of about 1075 F., i.e.between about 1050 and 1100 F. for 4 hours. Following this, the sheetswere flat and exhibited no coil The foregoing is a specific example ofthe processing steps for producing fiat heat-treated sheets, which maybe stated generally as follows:

(1) Hot rolling steel of suitable composition to band gage.

(2) Heating the strip to a suitable anstenitizing temperature (i.e.above the upper critical temperature of the steel).

(3) Quenching the strip at a rate sufficient to produce or to transformthe product to a martensitic or lower bainitic mi-crostructure.

(4) Tempering the strip at a temperature between 1000 F. and the lowercritical temperature of the steel.

(5) Cold rolling to the desired finished gage. If the band gage is morethan double the desired finished gage, the strip may be cold rolled toan intermediate size, reheated to a temperature not above the temperingtemperature and preferably about 50 to 100 F. lower than the firsttempering temperature, then cold rolled to the desired finished gage.The strip may be cold rolled and reheated as many times as are necessaryto effect reduction from band gage to finished gage while using 10% to50% reduction.

(6) Heating strip to a temperature above the recrystallizing temperaturebut below the temperature of tempering in Step 4.

(7) Cooling slowly to ambient temperature. If coil set is notappreciable, the process can end here. If coil set must be removed, thefollowing additional steps must be included.

(8) A light cold reduction, preferably under 2%.

(9) Reheating to temper at a temperature above the lower critical butabout 50 to 100 F. below the first tempering temperature as in the rangein Step 5 and cooling at such a rate as not to affect the flatness orother properties.

Another example of a low-alloy steel that may be used 5 in my inventionis one having a composition Within the ranges as stated above, exceptthat the nickel is omitted and the content of molybdenum is decreased toas little as 0.25%.

While I have shown and described several specific embodiments of myinvention, it will be understood that these embodiments are merely forthe purpose of illustration and description and that various other formsmay be devised Within the scope of my invention, as defined in theappended claims.

I claim:

1. A method of producing flat heat-treated steel sheets cornprisindrastically quenching from an austenitizing temperature a sheet ofhot-rolled steel capable of conversion to martensite or lower bainite totransform the same to a microstructure consisting substantially ofmartensite or lover bainite, such quenching producing distortiontherein, tempering said steel at a temperature above about 1000 F. andbelow its lower critical temperature, cold reducing said steel between10 and 50% to the desired thickness and then heating the cold-reducedsteel at a temperature above its recrystallization temperature but notabove said first-mentioned tempering temperature.

2. A method of producing fiat heat-treated low-alloy steel sheet of highstrength and toughness comprising drastically quenching a low-alloyhot-rolled steel sheet containing 0.10 to 0.30% carbon to transform thesame to a microstructure consisting substantially of martensite or lowerbainite, such quenching producing distortion therein, tempering saidsteel at a temperature above about 1000 F. and below its lower criticaltemperature, cold rolling said steel to reduce it between 10 and 50% tothe desired thickness and then heating the cold-reduced steel at atemperature above its recrystallization temperature but not above saidfirst-mentioned tempering temperature.

3. A method of producing fiat heat-treated low-alloy steel sheet of highstrength and toughness comprising drastically quenching a low-alloyhot-rolled steel sheet containing 0.10 to 0.30% carbon to transform thesame to a microstructure consisting substantially of martensite andlower bainite or mixtures thereof, such quenching producing severedistortion therein, tempering said steel at a temperature above about1000 F. and below its lower critical temperature, cold reducing saidsteel between 10 and 50% to the desired thickness and then heating thecold-reduced steel at a temperature above its recrystallizationtemperature but not above said first-mentioned tempering temperature.

References Cited by the Examiner UNITED ST TES PATENTS 3,053,703 9/62Breyer 148-12 FOREIG N PATENTS 782,356 9/57 Great Britain.

OTHER REFERENCES The Making, Shaping and Treating of Steel by U.S.S.,7th edition (pages 819-812 relied upon).

DAVID L. BECK, Primary Examiner.

1. A METHOD OF PRODUCING FLAT HEAT-TREATED STEEL SHEETS COMPRISINGDRASTICALLY QUENCHING FROM AN AUSTENITIZING TEMPERATURE A SHEET OFHOT-ROLLED STEEL CAPABLE OF CONVERSION TO MARTENSITE OR LOWER BAINITE TOTRANSFORM THE SAME TO A MICROSTRUCTURE CONSISTING SUBSTANTIALLY OFMARTENSITE OR LOWER BAINITE, SUCH QUENCHING PRODUCING DISTORTIONTHEREIN, TEMPERING SAID STEEL AT A TEMPERATURE ABOVE ABOUT 1000*F. ANDBELOW ITS LOWER CRITICAL TEMPERATURE, COLD REDUCING SAID STEEL BETWEEN10 AND 50% TO THE DESIRED THICKNESS AND THEN HEATING THE COLD-REDUCEDSTEEL AT A TEMPERATURE ABOVE ITS RECRYSTALLIZATION TEMPERATURE BUT NOTABOVE FIRST-MENTIONED TEMPERING TEMPERATURE.